Perturbation Training & Fall Prevention, Part I: Program Design
Traditional balance training often focuses on balancing on foam or unstable surfaces with eyes open or closed, applying physical perturbations such as push or pull at the shoulders or waist. However, our automatic responses to a slip or trip occur too quickly to be within our conscious control. An earlier blog post introduced the idea that these automatic responses can be improved with perturbation training that more closely resembles a slip or trip. While exercise programs decrease fall rate and risk of fall (ROF) by up to 30% (Grabiner, 2014), a later systematic review of community-dwelling elders found that exercise with high balance challenges had to be performed for more than three hours per week to show a 39% reduction in fall rate but lose the benefits when the exercise program stops. While generalized exercise programs can reduce fall risk, recovery strategies for falls are specific to the scenario in which they occur: slips and trips while walking, those that occur during sit to/from standing, descending stairs, or collisions. This suggests that specificity of training extends to including tasks that elicit automatic postural responses in program design perturbation training meets those demands.
Task-specific perturbation training (McCrum, 2022):
Must result in a sudden motor response and be large enough that the person would otherwise fall.
Must be like common perturbations experienced in everyday life.
Should be delivered while standing, walking, or other common movements.
Benefits may be limited to improvements in dynamic and perturbed balance tasks with little or no transfer to more static balance tasks.
Motor adaptations to perturbation training are either:
Predictive: Prior knowledge and experience feed forward to adjust the center of mass and base of support to reduce the impact of perturbation; gait training and generalized balance training target these predictive adaptations. If the anticipatory postural adjustments (hip and ankle strategies) are impaired and poorly coordinated, postural instability will be evident during self-initiated movements. Emotion and fear will affect these anticipatory postural adjustments and increase attentional demand to postural stability.
Reactive: With training, these pre-programmed responses are optimized to decrease the size of the muscular activity to maintain the center of gravity within the base of support, thus improving response efficiency. These automatic responses, typically occurring within the first 85-100 ms after the external perturbation, are assessed by the motor control test (MCT) and adaptation test (ADT). Perturbation training targets these very rapid response adaptations (e.g., stepping reactions, counter-rotation of the upper body and limbs, and grasping reactions) while other approaches (gait training, strength, balance, endurance) don’t specifically target these automatic reactions. If these pre-programmed responses are poorly coordinated, postural instability will be evident during the perturbation.
While the motor response latency remains unchanged with repeated perturbations, with practice the anticipatory postural responses shift from a hip strategy to an ankle strategy (Hill Et.al., 2018; Latash, 2012; Horak and Nashner, 1986). Following complete anterior cruciate ligament rupture, as few as 10 sessions of perturbation training improved dynamic knee stabilization, restored movement patterns, and decreased muscular co-activation (Chmielewski Et.al., 2005). Recent evidence suggests that the predictive neuromuscular response takes the form of reciprocal activation instead of a muscular co-activation strategy in some, but not all, tasks involving automatic postural control (Cesari, 2022). This is an area of ongoing research. In our next blog post, we’ll return to the four cases to illustrate the differences in perturbation training program design according to fall mechanism, medical considerations, and impairments.
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References
American College of Sports Medicine. (2020). ACSM's guidelines for exercise testing and prescription. 11th Ed. Lippincott Williams & Wilkins.
Cesari, P., Piscitelli, F., Pascucci, F., & Bertucco, M. (2022). Postural threat influences the coupling between anticipatory and compensatory postural adjustments in response to an external perturbation. Neuroscience, 490, 25-35.
Chmielewski, T. L., Hurd, W. J., Rudolph, K. S., Axe, M. J., & Snyder-Mackler, L. (2005). Perturbation training improves knee kinematics and reduces muscle co-contraction after complete unilateral anterior cruciate ligament rupture. Physical therapy, 85(8), 740-749.
Grabiner, M. D., Donovan, S., Bareither, M. L., Marone, J. R., Hamstra-Wright, K., Gatts, S., & Troy, K. L. (2008). Trunk kinematics and fall risk of older adults: translating biomechanical results to the clinic. Journal of Electromyography and Kinesiology, 18(2), 197-204
Hill, C. M., Wilson, S., Mouser, J. G., Donahue, P. T., & Chander, H. (2018). Motor adaptation during repeated motor control testing: Attenuated muscle activation without changes in response latencies. Journal of electromyography and kinesiology: official journal of the International Society of Electrophysiological Kinesiology, 41, 96–102. https://doi.org/10.1016/j.jelekin.2018.05.007
Horak, F. B., & Nashner, L. M. (1986). Central programming of postural movements: adaptation to altered support-surface configurations. Journal of neurophysiology, 55(6), 1369-1381.
Horak F. B. (2006). Postural orientation and equilibrium: what do we need to know about neural control of balance to prevent falls?. Age and ageing, 35 Suppl 2, ii7–ii11. https://doi.org/10.1093/ageing/afl077
Latash, M. L. (2012). Fundamentals of motor control. Academic Press.
McCrum, C., Bhatt, T. S., Gerards, M. H., Karamanidis, K., Rogers, M. W., Lord, S. R., & Okubo, Y. (2022). Perturbation-based balance training: Principles, mechanisms, and implementation in clinical practice. Frontiers in sports and active living, 4, 1015394.
Sherrington, C., Michaleff, Z. A., Fairhall, N., Paul, S. S., Tiedemann, A., Whitney, J., Cumming, R. G., Herbert, R. D., Close, J. C. T., & Lord, S. R. (2017). Exercise to prevent falls in older adults: an updated systematic review and meta-analysis. British journal of sports medicine, 51(24), 1750–1758. https://doi.org/10.1136/bjsports-2016-096547
